A toner image formed on an electrostatic or magnetic latent image is generally transferred to paper or a like medium and fixed thereon by heat fixing, solvent fixing, pressure fixing, etc. Pressure fixing, in which a toner image is fixed by application of pressure only, is advantageous in that an electric power consumption is small, a rapid start in fixing can be made, a high-speed fixing system can be applied, and the apparatus is simple.
On the other hand, however, fixing properties attained by pressure fixing is insufficient. That is, the pressure-fixed toner particles are apt to fall off the medium. An increase in applied pressure in an attempt to obtain a sufficient fixing level is attended by disadvantages such that transfer paper is damaged or becomes semi-transparent, the toner image becomes shiny, and the size of the fixing apparatus must be increased accordingly. The fixing level obtained for these disadvantages is still insufficient. In addition, a pressure-fixable toner contains a resin ready to be deformed on pressure application. This resin component tends to contaminate a photoreceptor or carrier particles used in a two-component developer or tends to deteriorate fluidity of the toner.
In order to solve these problems, capsule toners comprising microcapsules containing a pressure-fixable material, etc. have been developed. Since a pressure-fixable material is enclosed in capsule wall, it causes no contamination of a photoreceptor or carrier particles or no adverse influences on toner fluidity. Nevertheless, even capsule toners do not always achieve sufficient fixing performance properties depending on the characteristics of the pressure-fixable material to be used as a core material.
For example, where a wax as disclosed in JP-A-55-18654 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") or a liquid polymer as disclosed in JP-A-59-162562 is used as a pressure-fixable material, fixing properties achieved are not sufficient. While these pressure-fixable materials exhibit fixability, the fixed toner comes off by outer force with comparative ease, for example, when rubbed with fingers or when paper is superposed on the fixed toner image and letters are written thereon with a ballpoint pen. This is because the fixable component which is deformed or fluidized by pressure, such as a wax and a liquid polymer, retains its character of being deformed or fluidized even after being fixed.
A pressure-fixable material comprising a high-boiling solvent and a polymer as proposed in JP-A-58-145964 provides an excellent fixing level because the high-boiling solvent penetrates into paper or volatilizes after fixing to harden the pressure-fixable material. However, it takes time for the high-boiling solvent to penetrate into paper or to volatilize so that sufficient fixing strength cannot be obtained immediately after fixing.
For the purpose of obtaining a high fixing level or rapid fixing properties, JP-A-60-83958 discloses a pressure-fixable material having a disperse system in which a high-boiling solvent having dissolved therein a polymer forms a disperse phase and an organic liquid which is incapable of dissolving the polymer and incompatible with the high-boiling solvent forms a continuous phase. However, according to the structure disclosed, a stable disperse system cannot be formed. That is, the fixable material is separated into a polymer-containing high-boiling solvent phase and an organic liquid phase within the capsules during preservation, failing to maintain satisfactory fixability. Besides, since a solution of a polymer in a high-boiling solvent is used as a disperse phase, a sufficient fixing level cannot be reached until the high-boiling solvent volatilizes or penetrates into paper, i.e., instantaneous fixing cannot be achieved.
Although it is possible to reduce the time for reaching sufficient fixing strength by increasing volatility of a solvent in which a polymer is dissolved or dispersed, a system involving volatilization of a solvent, whether highly volatile or not, entails environmental pollution.
A core material comprising an organic solvent, a high polymer soluble in the organic solvent, and a pressure-fixable substance insoluble in the organic solvent is disclosed in JP-A-56-119137. According to this structure, a stable disperse system cannot be formed similarly to the above-mentioned technique, and the pressure-fixable substance undergoes sedimentation or agglomeration in the capsules during preservation. In addition, a sufficient fixing level cannot be reached because of the use of a wax type material as a pressure-fixable substance.
A core material comprising a non-aqueous solution or dispersion of a soft solid is disclosed in JP-A-51-124435. Even in the case of the non-aqueous dispersion, a stable toner film cannot be formed, and a sufficient fixing strength cannot be obtained instantaneously.
JP-A-59-159174 proposes a core material comprising two or more polymers having different glass transition temperatures. It is assumed from the method of preparation described, while not accounted for, that the two polymers are finely dispersed in the core, but the disperse state cannot be maintained in a stable manner.